The last Tory prime minister, John Major, was forced to sack Edward Leigh as trade minister over his opposition to the Maastricht Treaty - and Leigh is likely to prove no less troublesome to Cameron.
As president of the 40-strong Cornerstone Group of traditionalist Conservative MPs, Leigh has established himself as one of Cameron's most vociferous back-bench critics. With the Tories likely to win only a small majority, he will have a de facto power of veto over any contentious legislation.
Leigh, who supports the restoration of Section 28, is appalled by Cameron's social liberalism and is expected to renew attempts to reduce
the time limit on abortions. His chairmanship of the Commons public accounts committee has given him a forensic eye for detail and he is likely to lead opposition to Cameron's pledge to ring-fence the National Health Service budget. More recently, he has been touted by Tory MPs as a possible replacement for the Speaker of the Commons, John Bercow, in the next parliament.